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Use of cellulose nitrate in radiotoxicology for measurement of alpha low level activities in biological samples
Nuclear Tracks, Vol. 12, Nos I-6, pp. 809-811, 1986. Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
USE OF CELLULOSE NITRATE IN RADIOTOXICOLOGY FOR MEA~JREMENr OFALPHA LOW LEVEL ACTIVITIES IN BIOLOGICAL SAMPLES
L. Jeanmaire, M. Verry, C. Pitiot, A. Rannou C.E.A.-
I.P.S.N., D~partement de Protection Sanitaire, S.E.A.P.S., B.B. 6 - 92265 Fontenay aux Roses Cedex (FRANCE)
ABSTRACT The technique used for measurement of alpha activities below 4 mBq (0.1 pCi) in biological sample with cellulose nitrate (LR115) is described. After two months exposure, blanck activity of reactives is 0.1 mBq (3 fCi). The mean values for actual samples are about from three to five times higher than blank. KEYWORDS
Alpha measurement, plutonium, urine, feces, LR 115, cellulose nitrate. INTRODUCTION Measurement of alpha emitters at very low level activities is the goal of laboratories in charge of the surveillance of people working with transuranic radionuclides. These kinds of nucl~ des are generally not much transferable, and daily excretion lower than I pCi (37 mBq) is considered very significant from the health physics point of view. When the activity is lower than 4 mBq, counting time may require as much as 24 hours or more. lhusa technique which does not require expensive devices in~obilized during a long period may be of great interest if the results are reasonably acceptable for low level activities. For that purpose cellulose nitrate (LR 115 film - Kodak) has been tested as adetector in actual conditions. When a alpha particle of suitable energy passes through LR 115 (around 12 ~m thick) a hole appears after etching with sodium hydroxide solution. Knowing the number of hole produced and the detection yield, the alpha source activity can be determined. MATERIAL AND METHODS I. Procedure Samples are Lanthanum fluoride precipitates ( L a ~ 100 ug) obtained in different radiotoxicological laboratories (Jeanmaire et al., 1984). This form is the last step of chemical process for determination of plutonium in urine or feces, and of gross alpha in urine. This precipitate is filtered through Millipore filter.(@ 25 n~n);the diameter of deposits varies between 15 and 18 n~n. Millipore filters are then stuck on polyester holder having the same dimensions as film: v.s. 9 x 12 cm. Each sheet can receive 10 or 11 samples and blanks. A makrofol screen, I 0 ~m thickness is put on the samples, and a LR 115 film, type 2 stripping, covers the screen; for sample with activities lower than 7 mBq (0,2 pCi), exposure durations range between I and 4 months. After exposure,the detector is etched 140 minutes at 60 °C in a glass tube containing sodium hydroxide solution (2.5 N),without being agited. Then it is washed in running cold water for half an hour,and dried. Track counting may be carried out with microscope,but generally a spark counter is used after stripping the sensitive layer from the base. The detector yield measured with this procedure is about 10% forplutonium 239 and 238. 2. Cor~nents Ho£der Sar~jles holder must be free of alpha activity background;therefore some materials have to Be avo~4ed~ esDecially~Elass mld.Dap~r.;[email protected] han~d,Dlastic materials are often suitable, parzlcularl~ the polyeszer toil -wnlcn is ~ne Dase of I/I 115. 809
810
L. JEANMAIRE et a~.
S~een Screen (10 ~n) (Jeanmaire et aI,,1971) between samples and film reduces the energies of plutonit~n 239 and plutonium 238 to suitable values in view to obtain alpha holes during etching. If the alpha energies of radionuclides are not in the same range,with 244Cm for instance,the screen thickness must be adjusted,otherwise the detection yield becomes~ery low.
Processing Etching is the critical point when considering the reliability of the method. The slightest modification of the procedure or of the material can involve differences, Reproducibility can be checked by two ways: one by measuring the film thickness after etching and stripping; in the conditions described above results are satisfactory when thickness is about 7 um. The second one uses uranium thick sources that are handled like samples; etching conditions are adjusted in order to obtain constant yield. During three months, 16 films exposed on the same way to these test sources were etched and spark counted; the mean yield by film is 36.7 % + 3.4 (p = 0.95 for one film) showing a good stability; however the yield is not homogeneous ~ithin a film; it systematically decreases from top to bottom according to its position in the etching bath: 40.8 + 5.4 top, 36.0 + 4 middle, 33.1 + 4.2 bottom. Nevertheless,these results cannot be taken a~ standards. The-use of another water-bath, even of the same type,can lead to a different etching time in order to keep the same detection yield. Moreover the gradient of the yield according to the position within the film can be reversed. Various assays in order to reduce scattering on the same film have not succeed so far. To sur~narize,iftheprocessing is carried out exactly in the same waNthe results will be reploducible, but the slightest modification will introduce variations in the yield.
Spark eoun.tkng The method was described by Cross and Tonmmsino (1969) and modified by Le Thanh and Nickpay (1979). The counter consists of a kind of condenser where the dielectric is the c~llulose nitrate sheet with small holes due to alpha tracks; plates are aluminum foil 13 A thickness deposited on plastic sheet. When the high voltage (H.V.) applied to plates is high enough a spark flashes through a hole and volatilizes a s mall aluminum area on each plate. The short circuit discharges the condenser,giving an electric pulse which is counted by a scaler. After local pulverisation of aluminum, the short circuit stops and H.V. increases again up to a sufficient value to obtain a new spark in an other hole of the film. The actual device (fig. I) may count 12 samples. Electrodes are aluminized mylar strips, 20 nun wide, set in cross on both sides of the detector. The H.V. is applied on the X electrodes,theY electrodes collect pulses. Four horizontal and three vertical strips give a 12 elements matrix; all X,Y coordinates are scanned one after the other,which allows to read automatically the whole detector. The total counting time is set to 3 minutes for a sample,with 2 minutes at 500 V and I minute at 675 V. This time is long enough for most of the samples. Counting losses are negligible in these conditions when the track densities do not exceed 500 per square centimeter. RESULTS Over 1500 gross alpha samples have been measured with LR 115. They come from one radiotoxicological laboratory where they have been previously counted as usual. Each film is exposed to 10 or 11 samples. In some cases one or two results may be in a range well above the other ones and so cannot be considered as background samples. In these cases the mean is computed without taking in account these results by the way of rejecting values which are out of the mean plus two standard deviations, with the normal approximation for the distribution. Generally the mean thus computed for a film is in the range 0.3 - 0.6 mBq (8 - 16 fCi). This value is considered as the background of the technique. In actual conditions we admit that a raw value of 1.1 mBq (30 fCi) is really positive and could be connected with a net value of 0.5 to 0.8 mBq (15 - 22 fCi). This background value is easy to determine with LR 115; it appears as very different from the value of the lanthanum fluoride carrier 0.1 mBq (3 fCi),which itself is systematically higher than the background of the film. This last one does not grow significantly over one year period. It remains at 5-6 counts per cm2 for one deliyery. About 200 plutonium urine samples (fig. 2) come from an other laboratory. With the treatment of results the mean of a film is 0.3 mBq_+ 0.14 (8 fCi).
USE OF CELLULOSE NITRATE IN RADIOTOXICOLOGY
81
;OrN
•
Aluminized mylar strips
i
',0 '
oe
X Electrodes
._
ee oeo ooe
I
eeoe
20 -:=:: eoooo ooeoo oeooe oJooe
eoeoe oeooo o*eoo mooooo
I0-:::::: oee eeeoee eeeoeo
oe
• :
::::::.: Y Electrodes
Matrix for counting 12 samples
Exemple of selection: The counting area {shaded) is determined by the electrodes X=3, Y=2.
Fig. 1 - Matrix for counting 12 samples
eooooooe 'e~ee°°°~
o
•
:
•
•
0 :::::'::::~ . . . ~. : 4o ' :**~" "" Io 2o 5o 6o I'
0
''
'l''''l
I
2
L
,cia,,
I
I
mBq
3
Fig. 2 - Alpha activity measured with LR1]5 in 190 analysis of plutonium inurine
CONCLUSION With actual radiological samples without spike, LR 115 is useful for the measurement of plutonium low activities down to 0.6 mBq (20 fCi). If better chemical purification is obtained this film would pen lit to detect activities lower than 0.1 mBq. The film background being constant and exposure time not being restricted by price or number of measuring devices,the detection limit depends essentially on the sample background. REFERENCES
Cross, W.C. and Tor~msino, L. (1969). A rapid reading technique for nuclear particle damage tracks in thin foils, Jourentra~ Clermont-Ferrand~ 6-9 ~;~i. Jeanmaire, L. and Ballada, J. (1972). Assessment of radioactive contamination in man. Vienne, AIEA, 267- 268. Jearmmire, L., Verry, M., Pitiot, C. and Harduin, J.C. (1985). Assessment of radioactive contamination in man 1984. Vienne, AIEA, 519-524. Le Thanh, P. and Nickpay, P. (1979). Nuclear tracks suppl. 2, 337-362.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
USE OF CELLULOSE NITRATE IN RADIOTOXICOLOGY FOR MEA~JREMENr OFALPHA LOW LEVEL ACTIVITIES IN BIOLOGICAL SAMPLES
L. Jeanmaire, M. Verry, C. Pitiot, A. Rannou C.E.A.-
I.P.S.N., D~partement de Protection Sanitaire, S.E.A.P.S., B.B. 6 - 92265 Fontenay aux Roses Cedex (FRANCE)
ABSTRACT The technique used for measurement of alpha activities below 4 mBq (0.1 pCi) in biological sample with cellulose nitrate (LR115) is described. After two months exposure, blanck activity of reactives is 0.1 mBq (3 fCi). The mean values for actual samples are about from three to five times higher than blank. KEYWORDS
Alpha measurement, plutonium, urine, feces, LR 115, cellulose nitrate. INTRODUCTION Measurement of alpha emitters at very low level activities is the goal of laboratories in charge of the surveillance of people working with transuranic radionuclides. These kinds of nucl~ des are generally not much transferable, and daily excretion lower than I pCi (37 mBq) is considered very significant from the health physics point of view. When the activity is lower than 4 mBq, counting time may require as much as 24 hours or more. lhusa technique which does not require expensive devices in~obilized during a long period may be of great interest if the results are reasonably acceptable for low level activities. For that purpose cellulose nitrate (LR 115 film - Kodak) has been tested as adetector in actual conditions. When a alpha particle of suitable energy passes through LR 115 (around 12 ~m thick) a hole appears after etching with sodium hydroxide solution. Knowing the number of hole produced and the detection yield, the alpha source activity can be determined. MATERIAL AND METHODS I. Procedure Samples are Lanthanum fluoride precipitates ( L a ~ 100 ug) obtained in different radiotoxicological laboratories (Jeanmaire et al., 1984). This form is the last step of chemical process for determination of plutonium in urine or feces, and of gross alpha in urine. This precipitate is filtered through Millipore filter.(@ 25 n~n);the diameter of deposits varies between 15 and 18 n~n. Millipore filters are then stuck on polyester holder having the same dimensions as film: v.s. 9 x 12 cm. Each sheet can receive 10 or 11 samples and blanks. A makrofol screen, I 0 ~m thickness is put on the samples, and a LR 115 film, type 2 stripping, covers the screen; for sample with activities lower than 7 mBq (0,2 pCi), exposure durations range between I and 4 months. After exposure,the detector is etched 140 minutes at 60 °C in a glass tube containing sodium hydroxide solution (2.5 N),without being agited. Then it is washed in running cold water for half an hour,and dried. Track counting may be carried out with microscope,but generally a spark counter is used after stripping the sensitive layer from the base. The detector yield measured with this procedure is about 10% forplutonium 239 and 238. 2. Cor~nents Ho£der Sar~jles holder must be free of alpha activity background;therefore some materials have to Be avo~4ed~ esDecially~Elass mld.Dap~r.;[email protected] han~d,Dlastic materials are often suitable, parzlcularl~ the polyeszer toil -wnlcn is ~ne Dase of I/I 115. 809
810
L. JEANMAIRE et a~.
S~een Screen (10 ~n) (Jeanmaire et aI,,1971) between samples and film reduces the energies of plutonit~n 239 and plutonium 238 to suitable values in view to obtain alpha holes during etching. If the alpha energies of radionuclides are not in the same range,with 244Cm for instance,the screen thickness must be adjusted,otherwise the detection yield becomes~ery low.
Processing Etching is the critical point when considering the reliability of the method. The slightest modification of the procedure or of the material can involve differences, Reproducibility can be checked by two ways: one by measuring the film thickness after etching and stripping; in the conditions described above results are satisfactory when thickness is about 7 um. The second one uses uranium thick sources that are handled like samples; etching conditions are adjusted in order to obtain constant yield. During three months, 16 films exposed on the same way to these test sources were etched and spark counted; the mean yield by film is 36.7 % + 3.4 (p = 0.95 for one film) showing a good stability; however the yield is not homogeneous ~ithin a film; it systematically decreases from top to bottom according to its position in the etching bath: 40.8 + 5.4 top, 36.0 + 4 middle, 33.1 + 4.2 bottom. Nevertheless,these results cannot be taken a~ standards. The-use of another water-bath, even of the same type,can lead to a different etching time in order to keep the same detection yield. Moreover the gradient of the yield according to the position within the film can be reversed. Various assays in order to reduce scattering on the same film have not succeed so far. To sur~narize,iftheprocessing is carried out exactly in the same waNthe results will be reploducible, but the slightest modification will introduce variations in the yield.
Spark eoun.tkng The method was described by Cross and Tonmmsino (1969) and modified by Le Thanh and Nickpay (1979). The counter consists of a kind of condenser where the dielectric is the c~llulose nitrate sheet with small holes due to alpha tracks; plates are aluminum foil 13 A thickness deposited on plastic sheet. When the high voltage (H.V.) applied to plates is high enough a spark flashes through a hole and volatilizes a s mall aluminum area on each plate. The short circuit discharges the condenser,giving an electric pulse which is counted by a scaler. After local pulverisation of aluminum, the short circuit stops and H.V. increases again up to a sufficient value to obtain a new spark in an other hole of the film. The actual device (fig. I) may count 12 samples. Electrodes are aluminized mylar strips, 20 nun wide, set in cross on both sides of the detector. The H.V. is applied on the X electrodes,theY electrodes collect pulses. Four horizontal and three vertical strips give a 12 elements matrix; all X,Y coordinates are scanned one after the other,which allows to read automatically the whole detector. The total counting time is set to 3 minutes for a sample,with 2 minutes at 500 V and I minute at 675 V. This time is long enough for most of the samples. Counting losses are negligible in these conditions when the track densities do not exceed 500 per square centimeter. RESULTS Over 1500 gross alpha samples have been measured with LR 115. They come from one radiotoxicological laboratory where they have been previously counted as usual. Each film is exposed to 10 or 11 samples. In some cases one or two results may be in a range well above the other ones and so cannot be considered as background samples. In these cases the mean is computed without taking in account these results by the way of rejecting values which are out of the mean plus two standard deviations, with the normal approximation for the distribution. Generally the mean thus computed for a film is in the range 0.3 - 0.6 mBq (8 - 16 fCi). This value is considered as the background of the technique. In actual conditions we admit that a raw value of 1.1 mBq (30 fCi) is really positive and could be connected with a net value of 0.5 to 0.8 mBq (15 - 22 fCi). This background value is easy to determine with LR 115; it appears as very different from the value of the lanthanum fluoride carrier 0.1 mBq (3 fCi),which itself is systematically higher than the background of the film. This last one does not grow significantly over one year period. It remains at 5-6 counts per cm2 for one deliyery. About 200 plutonium urine samples (fig. 2) come from an other laboratory. With the treatment of results the mean of a film is 0.3 mBq_+ 0.14 (8 fCi).
USE OF CELLULOSE NITRATE IN RADIOTOXICOLOGY
81
;OrN
•
Aluminized mylar strips
i
',0 '
oe
X Electrodes
._
ee oeo ooe
I
eeoe
20 -:=:: eoooo ooeoo oeooe oJooe
eoeoe oeooo o*eoo mooooo
I0-:::::: oee eeeoee eeeoeo
oe
• :
::::::.: Y Electrodes
Matrix for counting 12 samples
Exemple of selection: The counting area {shaded) is determined by the electrodes X=3, Y=2.
Fig. 1 - Matrix for counting 12 samples
eooooooe 'e~ee°°°~
o
•
:
•
•
0 :::::'::::~ . . . ~. : 4o ' :**~" "" Io 2o 5o 6o I'
0
''
'l''''l
I
2
L
,cia,,
I
I
mBq
3
Fig. 2 - Alpha activity measured with LR1]5 in 190 analysis of plutonium inurine
CONCLUSION With actual radiological samples without spike, LR 115 is useful for the measurement of plutonium low activities down to 0.6 mBq (20 fCi). If better chemical purification is obtained this film would pen lit to detect activities lower than 0.1 mBq. The film background being constant and exposure time not being restricted by price or number of measuring devices,the detection limit depends essentially on the sample background. REFERENCES
Cross, W.C. and Tor~msino, L. (1969). A rapid reading technique for nuclear particle damage tracks in thin foils, Jourentra~ Clermont-Ferrand~ 6-9 ~;~i. Jeanmaire, L. and Ballada, J. (1972). Assessment of radioactive contamination in man. Vienne, AIEA, 267- 268. Jearmmire, L., Verry, M., Pitiot, C. and Harduin, J.C. (1985). Assessment of radioactive contamination in man 1984. Vienne, AIEA, 519-524. Le Thanh, P. and Nickpay, P. (1979). Nuclear tracks suppl. 2, 337-362.